A so-called “gear-change shock” has been a problem in a conventional automatic transmission apparatus installed in a vehicle. An impact is generated when a gear ratio is changed in the automatic transmission apparatus and a vehicle driver feels the impact as the gear-change shock. In a known oil pressure control apparatus for the automatic transmission apparatus, oil pressure of working oil which is supplied to respective gear-change elements for changing the gear ratio is changed in terms of time in accordance with a predetermined control pattern (also referred to as an oil pressure pattern) in order to reduce the gear-change shock.
In the known automatic transmission apparatus, the oil pressure control apparatus has a mechanical pump, electromagnetic valves, an electronic control unit (hereinafter also referred to as TCU “Transmission Control Unit”) and so on.
The mechanical pump is driven by an internal combustion engine to generate oil pressure of working oil. The electromagnetic valve adjusts the oil pressure generated by the mechanical pump and outputs such adjusted oil pressure to gear-change elements of the automatic transmission apparatus. The TCU detects a vehicle condition based on signals from various kinds of sensors and controls various kinds of equipment related to an operation of the automatic transmission apparatus (including the electromagnetic valves) in accordance with such information relating to the vehicle condition. The TCU has a micro-computer and various kinds of driver circuits.
The TCU calculates command values for the oil pressure to be applied to the respective gear-change elements based on the oil pressure pattern. The TCU further calculates, based on the command values for the oil pressure, command values for drive currents to be supplied to respective driver circuits, which drive corresponding electromagnetic valves. The oil pressure is controlled when the drive current is supplied from the respective driver circuit to each of the electromagnetic valves. In addition, the TCU calculates the command values for the drive current based on a current-command characteristic, which is prepared in advance.
The current-command characteristic is a correlation between the command value for the drive current to be supplied to the driver circuit for driving the electromagnetic valve and the command value for the oil pressure to be applied to the gear-change element.
The current-command characteristic is prepared in advance, for example, in the following process. In an initial condition before shipping products from a factory, necessary data relating to the drive current and the oil pressure for each of the electromagnetic valves and the gear-change elements are collected by changing the command value for the drive current supplied from the TCU to the driver circuit for driving the electromagnetic valve and by detecting the oil pressure of the working oil applied to the gear-change element. The current-command characteristic is prepared based on the above data for each of the electromagnetic valves and the gear-change elements (for example, as disclosed in Japanese Patent Publication No. 2001-116130).
The gear-change shock is caused by the impact generated when changing the gear ratio. Therefore, the gear-change shock may be generated, even when a small pressure difference of “tens of kPa” is generated between an actual value of the oil pressure applied to the gear-change element and the command value for the oil pressure. It is, therefore, quite a challenging target to completely solve the problem of the gear-change shock.
With regard to the oil pressure applied to the gear-change elements, the pressure difference of “tens of kPa” between the actual value and the command value may be substantially generated because of the following reasons (the pressure difference is also referred to as a deviation of the oil pressure). Not only a correlation between a thrust force for a spool and an opening degree of a valve member in the electromagnetic valve but also a characteristic of electric circuits including a coil and a driver circuit of the electromagnetic valve is inevitably deteriorated with age due to its use, even though deterioration is small and the deterioration gradually goes. As a result of such inevitable deterioration with age, the pressure difference (the deviation of the oil pressure) of “tens of kPa” may be substantially generated.
It is, therefore, desired to take any possible measures to reduce the pressure difference (the deviation of the oil pressure) so as to suppress the gear-change shock, although it is difficult to completely eliminate the gear-change shock.
For example, a feedback of the oil pressure, an updating of the current-command characteristic or the like, as explained below, may be possible counter measures for reducing the pressure difference.
According to the above-mentioned feedback of the oil pressure, for example, the oil pressure outputted from the electromagnetic valve is detected by an oil-pressure sensor and feed-backed to the TCU in order to increase or decrease the command value for the drive current (for example, as disclosed in Japanese Patent Publication No. 2004-205044).
However, it is not avoidable that a response delay with respect to the command, which may be caused by viscosity of the working oil, is generated, even when the opening degree of the valve member of the electromagnetic valve is changed by increasing or decreasing the command value for the drive current. Accordingly, it is not possible to obtain a sufficient effect for reducing the pressure difference (the deviation of the oil pressure) by the feedback of the oil pressure.
According to the above-mentioned updating of the current-command characteristic, data relating to actual values of the oil pressure are obtained during an operation of the automatic transmission apparatus and the current-command characteristic is updated based on the above data and the command value for the drive current. The data relating to the actual values of the oil pressure include, for example, detection values of the oil-pressure sensor, in a case that the oil-pressure sensor is provided.
However, the data obtained during the operation of the automatic transmission apparatus are those data obtained when the automatic transmission apparatus is in its transit period. As a result, the updated current-command characteristic based on the actual values of the oil pressure in the transit period may become lower in its reliability.
In addition, since the detection values for the oil pressure are obtained during the operation of the engine, an un-intentional operation of the automatic transmission apparatus may be caused if the oil pressure of the working oil is changed. Therefore, it is not always a recommendable method.
In addition, in a case that a shift range position is in an N-range or a P-range, the oil pressure is generally not applied to the electromagnetic valve. Therefore, it is not possible to obtain the data relating to the actual values of the oil pressure in such shift range positions.